As semiconductor device features continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers at various stages throughout a semiconductor fabrication process. One such non-destructive method includes optical based coherence probe microscopy. Coherence probe microscopy involves measuring the mutual coherence between a light wave reflecting from a reference object (e.g., reference mirror) and a light wave reflecting from the surface of a specimen using a two-beam interference based microscope. In a typical coherence probe measurement, a pin diode array (PDA) covering a portion of a field of view (FOV) may be utilized to detect the coherence of light reflected from an area of a surface of a specimen to be inspected. In this manner, a CCD camera may be utilized to image the surface of a given specimen, while the PDA is utilized for coherence analysis.
Autofocusing in such a system may be achieved by scanning the image in the Z-direction (i.e., along the primary optical axis) which provides interference intensity (as measured by the PDA) versus Z-position information. The interference intensity information may then be analyzed using software executed by a processor (e.g., computer system) communicatively coupled to the PDA in order to determine the best focus position. In this manner, the Z-position dependence of the intensity of interference fringes created by the superposition of light waves from a reference path and an object path of the two-beam microscope may be measured and analyzed. The Archer 100 tool manufactured by KLA-Tencor represents a system capable of carrying out traditional autofocusing in a two-beam interference microscope configuration.
In a traditional autofocusing two-beam interference microscopy arrangement, the CCD camera typically images the surface of the specimen only after autofocusing has been achieved utilizing the PDA and an associated computer system. In order to carry out CCD imaging following an autofocusing process, a controllable shutter system is positioned in the reference path. In this manner, the light beam reflecting from the surface of a reference mirror may be selectably blocked using the shutter system. Upon closing the shutter, the interference fringes will cease to appear at the CCD imaging plane as light from the reference path is no longer allowed to interfere with light from the object path. The camera may then image the surface of the specimen without the appearance of interference fringes in the field of view (FOV) of the camera.
The above described autofocusing and imaging process, however, requires a time delay. The CCD measurement may only image the surface of the specimen after the shutter system ceases to move and after residual vibrations within the system have stopped. For example, a system may require up to 40 to 50 milliseconds for shutter related vibrations to dissipate to a level required for subsequent CCD imaging. When integrated over an entire sampling process, this time delay may be significant. In some instances, the time delay required for shutter related vibrational noise to leave the system may represent up to 10% of the move-acquire-measure (MAM) time associated with a given measurement process.
It is therefore desirable to provide a two-beam interference autofocusing system capable of performing CCD imaging measurements without the need of a reference beam shutter.